The invention relates to a process for the production of biaxially stretched films having surfaces of predetermined roughness from a plastic melt which are extruded from a slot die, cooled on a roll into an intermediate film, then biaxially stretched and heat-set, and to an apparatus for implementation of the process.
Heat sealable films of polypropylene which are oriented in one or more directions are described in German Patent Specification No. 1,694,547. Owing to their mechanical strength, optical clarity and good water vapor impermeability, such polypropylene films are wellsuited for packaging purposes and can likewise also be used for electrical applications as capacitor films. Examples of this are electrical insulating films in metallized form for the production of capacitors. Films for electrical insulating purposes essentially have to meet the following two requirements. They must be suitable for further processing and possess the requisite electrical properties.
In the further processing of such films on high-speed machines it has been found that smooth polypropylene films have a tendency to block during metallizing, cutting and winding. In order to avoid this phenomenon, generally polypropylene films with rough surfaces are produced, such as are described in the publications of German Patent No. 25 53 693, German Offenlegungsschriften No. 26 01 810 and No. 27 40 237, and these are well-suited to further processing.
German Patent No. 25 53 693 describes a process for the production of a translucent polypropylene film having a net-like surface on one side, in which a molding compound of polypropylene is extruded without .beta.-crystallization-promoting nucleation to give an intermediate film which, after cooling, is heated to stretching temperature and biaxially stretched directly thereafter. By maintaining precise stretching temperatures and conditions and predetermined roll/water temperatures on the film surface, a fibrillated network structure is produced. This network structure better absorbs the insulating oils used in the production of capacitors. The condition of the other film surface, in direct contact with a roll, is undefined in this process.
German Offenlegungsschrift No. 26 01 810 discloses a blown film extrusion process for the production of a thermoplastic film, in which the molten thermoplastic material is extruded through a die-plate into a parison. For crystallization, the parison is taken coaxially over a cooling pin, a constant elevated temperature being maintained throughout on the outside of the parison in a predetermined position between the dieplate, the so-called frost line, in order to bring about a skin effect extending over the outside of the parison. An irregular surface of the parison is produced, facilitating winding and coiling by virtue of its tack-free or non-adhesive properties. The roughness of the other parison surface again remains undefined in this case.
German Offenlegungsschrift No. 27 40 237 describes a process for the production of a polypropylene film with a rough surface which has good electrical properties and an improved surface slip compared with films according to the prior art. For production of the intermediate film during pouring of the melt, the crystallization temperature of the polymer is set to a value .ltoreq.80.degree. C./sec and the interval within which the surface temperature of the melt lies in the range between the crystallization temperature and a temperature 20.degree. C. lower is set to at least 1/3 second. In other words, this means that the rate of cooling of the melt at crystallization temperature must not exceed a certain value, and a certain interval has to be maintained about the crystallization point of the melt. In this process, the thermal state of the film surface with respect to the draw-off roll is defined, and the desired roughness values can be adjusted there. The temperature of the film surface as it is unwound from the roll surface depends not only on the roll temperature but also very much on the melt film thickness and the circumferential speed of the roll. The roughness of this film surface on the air side is thus not adjustable in a defined way but is more the arbitrary result of the parameters mentioned above.
The known processs have in common that they are very susceptible to external influences, such as, for example, air convection, and, owing to the process conditions described, only one defined roughness is produced for one single film surface.
It is found when investigating the electrical properties of biaxially stretched polypropylene films that these electrical properties are better with smooth, flat surfaces than with rough surfaces. In particular, the so-called lifetime test of polypropylene films, which are used as insulating films in capacitors where the loss in capacitance of a capacitor is measured as a function of time .DELTA.C/C=f(t), shows a clear dependence on the surface roughness. On the other hand, smooth films create the previously mentioned problem of blocking which occurs during the further processing.